7.4.3　Construction Risk Analysis and Countermeasur...

7.4.3　Construction Risk Analysis and Countermeasures of River Channel Section

When the shield tunneling in the shallow soil area of the channel section under the water area，due to the unbalanced force on the upper and lower parts of the shield，the attitude of the shield is upward，the crushing is difficult，the tunnel is floating and the axis is difficult to control.After the assembled tunnel ring is removed from the tail of the shield，when the earth pressure on the upper part of the segment and the weight of the segment cannot resist the buoyancy of the surrounding groundwater or the grouting slurry at the tail of the shield，the tunnel will float and cause passive damage to the soil above the segment.Therefore，it is necessary to analyze the possible construction risks of shield tunneling in the middle of the Yangtze River，and formulate corresponding control measures and emergency plans.

1）Risk analysis and countermeasures

Taking the shield construction of Nanjing Yangtze River Tunnel crossing the middle channel section as an example.

（1）The overburden layer of the punching section is very shallow.The thickness of the overburden layer of 72 m punching section is less than twice the shield diameter，and the minimum thickness is 11.31-12.34 m，only 0.78 times the shield diameter.The stratum has hydraulic connection with the river water，which is prone to roof fall and collapse.

（2）The vertical direction of the punching segment shield is 4.5％upward slope，and the excavation face has 4.5％inclination，which is easy to lose stability.

（3）The horizontal direction of the trench shield is R=2500 m horizontal curve segment，the curve segment shield will inevitably cause overbreak，which is not conducive to the stability of the excavation face.In addition，due to the non-synchronization of shield and segment steering in the curve section，the shield tail clearance is difficult to control，and the segment gap is prone to unevenness，resulting in water leakage，slurry leakage or segment shelling at the shield tail.

（4）The upper soil of the punching section is silty sand layer，and the liquefaction grade is mild liquefaction to medium liquefaction.When it is disturbed by shield tunneling，it is prone to liquefaction and collapse.When the shield tail sealing effect is not good or the grouting pipe is cleaned，water gushing and sand gushing may occur.

In view of the above four points，the following countermeasures are adopted for construction control：

（1）In order to prevent the formation splitting caused by long-term shutdown of the shield in this section，a comprehensive inspection of the shield is carried out before the shield enters this section.All the spare parts with hidden dangers are replaced，and the comprehensive overhaul and replacement of the assembly system are completed.A high-voltage cable is replaced before entering the groove section，so as to ensure that the groove section is promoted in accordance with the principles of high viscosity and heavy slurry，reasonable low pressure，smooth propulsion，rapid assembly，prohibition of shutdown and one-pass，and strive to shorten the crossing time.The high quality slurry was prepared in the slurry tank，and 400 m3 new slurry was kept in the storage tank for maintenance of excavation face and slurry adjustment at the end of each ring.Reserve 3100 m3 slurry in the mixing tank to meet the requirements of the index，and reserve 5000 m3 slurry in the waste slurry tank.On-site storage room reserves not less than 20t NSHS-1，10t NSHS-2，20t NSHS-3，10t NSHS-4 and other polymer materials and 50t bentonite，storage room reserves 100t NSHS-1，30t NSHS-2，100t NSHS-3，50t NSHS-4 and other polymer materials and 200t bentonite.Drivage using high density，high viscosity mud to form dense mud film，closed excavation face.The mud pressure is calculated according to the parameters of weak strata，providing sufficient support pressure for the excavation face.At the same time，the mud pressure and grouting pressure（fluctuation±0.1 bar）are strictly controlled to prevent the pressure from breaking through the overburden.In order to ensure the accuracy of tunneling parameters，a field measurement of the topographic section of the bottom of the river was carried out before passing through the gully section.During the crossing process，the water level was measured by special person before each ring tunneling，and the tunneling parameters were corrected.Strengthen the condition monitoring and maintenance of equipment in the process of crossing，equipped with necessary vulnerable parts.

（2）4.5％of the slope of the excavation face is difficult to maintain stability when the excavation stops，so the mud must have the ability to rapidly film.According to the empirical data，combined with the calculation pressure，some mud pressure should be appropriately increased to offset the instability of the excavation face caused by the influence of the slope.Therefore，80 m3 high concentration new slurry must be injected into the excavation face in time before the end of each ring excavation to maintain the stability of the excavation face.

（3）In order to reduce the influence of the plane curve section on the construction of the chute section，the uniform steering is carried out according to the design offset of each ring during tunneling，so as to prevent the phenomenon of rapid turning.In order to ensure that the shield tail clearance does not deteriorate，each ring is divided into 0.5 m，1 m，1.5 m，2 m four mileage segments of the shield tail clearance selected 10 points for measurement，and according to the measured data of the shield tail clearance，in the process of excavation，timely slow correction，strictly prohibit excessive correction.The segment selection is mainly based on machine selection and manual review，and the segment installation form that is most matched with the shield machine attitude is selected.When slurry leakage is caused by uneven shield tail clearance，the water-stop sponge can be pasted on the outer arc surface of the segment in order to block the water absorption expansion when slurry leakage occurs at shield tail.At the same time，the oil injection amount can be increased with the increase of synchronous grouting amount.

（4）For the liquefaction of sand layer，the disturbance is minimized by reducing the tunneling speed and the rotational speed of the cutterhead in the tunneling parameters.The dense mud film was formed by using high density and high viscosity mud，and the excavation surface was closed.At the same time，through the mud film formation process，the viscous particles are penetrated into the silty sand layer，which can also improve the liquefaction phenomenon of the silty sand layer and eliminate the liquefaction effect of the soil contacted with the cutterhead.According to the parameters of fine sand layer，the slurry pressure is calculated to provide sufficient support pressure for the excavation face.In view of the possible water and sand gushing phenomenon，on the one hand，it is necessary to ensure that the shield tail gap is uniform.On the other hand，it is necessary to ensure that the injection amount of back wall grouting and shield tail sealing oil is sufficient.At the end of each grouting，the mixed slurry of fly ash and bentonite is used to keep pressure on the shield tail grouting pipe to prevent the inflow of external mud.

2）Grouting measures at river bottom

The main countermeasures adopted in the occurrence of river bottom slurry fall are as follows：

（1）When the river bottom slurry is found，it is necessary to closely monitor the liquid level display of the main control room and the variation of the slurry flow rate and dry sand volume.If the slurry is slight and can be pushed forward without reducing the water pressure of the excavation surface，it will continue to move forward.At the same time，it is necessary to accelerate the propulsion speed appropriately.NSHS-2 plugging material is injected from the emergency grouting mouth.When the machine is stopped and assembled，the slurry pressure is appropriately reduced，and the plugging material is injected.At the same time，the jack thrust is appropriately increased to maintain the supporting effect of the cutterhead on the excavation surface，improve the assembly efficiency and shorten the assembly time，so that the shield can pass through the slurry area as soon as possible.

（2）When the slurry burst is serious and cannot be pushed forward，the slurry pressure is gradually reduced to+0.4 bar of groundwater pressure，maintaining the balance of excavation face，further improving the density and viscosity of slurry，and adding 2％-4％NSHS-2 plugging agent to the slurry to block the position of slurry burst.The slurry flow is moderately increased by using the reserved slurry，and the smooth propulsion is adopted to shorten the assembly time and quickly pass through the slurry burst area.

（3）When it is found that the river water flows into the tunnel from the tail of the shield，the current situation should be first analyzed and the force should be organized for emergency.In the emergency process，the cement mortar and high quality shield tail oil（WR90）with short setting time（3-4 h）were injected at the leakage site.At the same time，the existing sewage pump（200 m3/h+200 m3/h）is used to do the drainage work，and the pump is added according to the needs to ensure the smooth discharge of the river into the shield tunnel.

3）Emergency plan

Due to the great risks in the construction of shield crossing the middle channel section of the Yangtze River，the following engineering safety plans are formulated in combination with the actual situation of Nanjing Yangtze River Tunnel.

（1）Technical measures.

This paper discusses and analyzes all kinds of dangerous sources that may occur when shield passes through the middle channel of the river，and formulates corresponding countermeasures，as shown in Table 7.6.

The specific operation control points are as follows：

①Ground monitoring should be timely feedback to the duty leader，technical personnel and shield，slurry field operators.

②If the river surface slurry burst occurs，it indicates that the mud pressure has broken through the overburden.At this time，the hydraulic connection between the river water and the mud tank has occurred，and a large number of mudleakage and soil erosion may occur.At this time，it is necessary to appropriately reduce the mud pressure，increase the mud density，and increase the tunneling speed，so that the shield can quickly cross the slurry burst area.After the shield passes through the grouting area，the machine is stopped to supplement grouting and the through joint is closed.

③If the river water pours into the tunnel，it indicates that there is a partial failure of the shield tail seal or the segment water stop.At this time，it is necessary to carry out strong drainage to prevent the shield components from being soaked in water，and to identify the leakage position.For example，the segment leakage can be treated by conventional concrete plugging technology such as polyurethane injection.For example，the shield tail leakage can start the shield tail emergency sealing device，increase the amount of oil and synchronous grouting，so as to play the role of waterproof and waterproof.

④If the excavation face is unstable or collapsed，it indicates that the mud film on the excavation face has failed，and the mud support pressure is less than the external water and soil pressure.In this case，the ground prefabricated high viscosity and high density new slurry should be used to replace the slurry on the excavation face immediately，and the support pressure should be increased to restore the stability of the excavation face.Because the excavation face of the collapsed stratum is not complete，it is necessary to make the shield tunneling slowly through the collapsed area and reestablish the balance after the slurry replacement is completed.

⑤In order to ensure the smooth implementation of the above measures，a comprehensive inspection of all equipment，materials and personnel is arranged before the shield passes through the river channel section.

（2）Rescue measures.

①When a sudden accident occurs，the on-duty leader，technical personnel or security personnel on the spot should immediately organize unrelated personnel to evacuate the dangerous area quickly，and the machinery that cannot be evacuated immediately should not be evacuated and people are the main force to ensure the safety of the construction personnel.

②After the evacuation of dangerous places（generally evacuated to the hole），immediately count the number of site construction personnel，check whether the personnel have not escaped from the scene，and immediately report the relevant situation to the project department leader.

③After receiving the notification，the leaders of the project department should immediately start the emergency relief program，organize manpower and material resources to make full efforts to rescue and relief，and reduce disaster losses.

④When casualties occur，according to the emergency rescue plan timely rescue work.Rescue should be carried out to ensure that rescue workers have no life and safety threats.If they have no rescue ability，they should report to the local government or relevant departments in time for rescue，and cooperate with the rescue work.

⑤When rescuing the wounded，according to the number of wounded and the degree of injury，the medical staff should take corresponding emergency measures at the scene.According to the principle of“first heavy and then light，”the wounded were sent to the hospital for rescue and treatment in time.

⑥On-site isolation measures such as safety warning lines are adopted to prevent other personnel from entering dangerous areas and avoid the expansion of disaster losses.

⑦According to the situation of disaster losses，in accordance with the relevant laws，regulations and provisions of the national and local governments at all levels，timely reporting to higher authorities or relevant departments，pending further investigation and processing.

（3）Logistics support measures.

Field commanders should be equipped with intercoms to ensure smooth communication and communication.

A sufficient amount of emergency materials such as straw bags，woven bags，polyurethanes and cotton quilts，and equipment such as pumps，excavators and slag trucks，including stretchers，were prepared on the site before the shield passed through the middle reaches of the Yangtze River.The pump is installed in place，and the pipeline is connected.Once the water gushes，it can be discharged immediately，and the slag truck is equipped with clay for standby.Site communications，lighting，drainage，ventilation，fire and other equipment to be configured to prevent the occurrence of dangerous scene chaos.In addition，all personnel are required to wear safety hats，water shoes，life jackets and other labor protection products to protect the life safety of workers.